When a cellulosic starting material is treated in the presence of 2,2,6,6-tetramethyl-1-piperidin-N-oxyradical (hereinafter “TEMPO”) and sodium hypochlorite which is an inexpensive oxidizing agent, carboxyl groups can be introduced efficiently into the cellulose microfibril surface. It is known that when the cellulosic starting material into which the carboxyl groups have been introduced is treated with a mixer or the like in water, a high-viscosity and transparent aqueous dispersion of cellulose nanofibers can be obtained (Non-patent Document 1, Patent Documents 1 and 2).
Cellulose nanofibers are a novel water-dispersible material that is biodegradable. Since carboxyl groups are introduced in the surface of cellulose nanofibers by oxidation reaction, the quality of cellulose nanofibers can be modified using the carboxyl groups as base points. Further, since the cellulose nanofibers obtained by the method mentioned above are in the form of a dispersion liquid, the quality can also be modified by blending the cellulose nanofibers with various water-soluble polymers or forming a composite of the cellulose nanofibers with an organic and/or inorganic pigment. Furthermore, the cellulose nanofibers can also be formed into a sheet or fiber. These characteristics of cellulose nanofibers are expected to be of help in making highly functional products obtained by application of cellulose nanofibers as an environmentally recyclable material to highly functional packaging materials, transparent organic substrate materials, highly functional fibers, separation membranes, regenerative biomaterials and the like.